Battery not charging well on sunny days

Had something similar last summer with my static. Sunny days, panels looking great, but the Victron MPPT was barely pushing anything into the batteries by mid-morning. Figured it was a wiring gremlin at first.

Turned out to be absorption voltage set too low on the charger profile. Battery was hitting that threshold dead early and just... sitting there doing nothing useful. Classic rookie mistake on my part, took me embarrassingly long to spot it.

Few things worth checking if you're in the same boat:

• State of charge when you start the day — if batteries are nearly full already, charging will naturally taper off
• Absorption/float voltage settings — these need to match your battery chemistry exactly. LiFePO4 vs AGM are totally different
• Cable sizing and connections — dodgy crimp or undersized cable will throttle everything even in perfect sun
• Panel temperature — counterintuitively, panels actually perform worse in hot weather. Output drops a fair bit on a scorching day

What battery are you running? Fogstar Drift or similar LiFePO4? Or older lead acid? Makes a massive difference to how you'd diagnose this.

Also worth chucking the Victron app on your phone if you haven't — the history graphs on the MPPT are dead useful for spotting exactly when charging drops off and why.

Don't be put off by not knowing the terminology, everyone started somewhere. Just post your setup specs if you can and people here will help figure it out.

@ZFS_OffGrid did you rule out temperature compensation being off? My Victron MPPT on the motorhome was doing exactly this — cracking sunny days but charge dropping off early. Turned out the absorption voltage was being hit sooner than expected because the battery temp sensor wasn't connected properly, so it was assuming a higher ambient temp and pulling back the target voltage.

Worth checking in the VictronConnect app — what state is it actually reporting when it backs off? If it's showing "absorption" or "float" already by mid-morning, the bank might genuinely be fuller than you think, or the voltage settings are wrong for your battery chemistry. What batteries are you running?

@ZFS_OffGrid mine did this exact thing — turned out the batteries were already full by 10am and the MPPT was doing its job perfectly, which felt deeply insulting after I'd spent three weekends rewiring everything like a deranged electrician.

Hey @ZFS_OffGrid — worth checking if your absorption voltage is set correctly for your battery chemistry. I had something almost identical and it turned out my bulk/absorption threshold was a touch low, so the MPPT was hitting "done" far too early and dropping back to float before the batteries were properly topped up. Looked like a charging problem but was actually just a settings issue. Pop into VictronConnect and have a look at your charge profile — if you're running AGMs or lithiums the defaults sometimes need tweaking to match your specific batteries. What battery type are you running? That'd help narrow it down.

@ZFS_OffGrid one thing nobody's mentioned yet — have you checked whether your MPPT is going into thermal derating? On a hot sunny day the controller itself can overheat, particularly if it's mounted in an enclosed space or directly in sunlight, and it'll deliberately throttle output to protect itself. The Victron MPPT units log this in VictorConnect under the "history" tab — look for any entries flagged as high temperature warnings.

Also worth pulling the full charge cycle data from VictorConnect and checking what state of charge the batteries were actually at when output dropped off. Correlating panel voltage, battery voltage, and charge current together often reveals the culprit far more reliably than chasing individual symptoms. I had a very similar pattern on my setup before I realised the controller was mounted on a south-facing timber panel — moving it to a shaded internal wall sorted it completely.